Quadrature amplitude modulation of optical carriers

ABSTRACT

An apparatus includes an optical splitter, an optical combiner, first and second optical paths, and a digital signal generator. The optical splitter has an input port and first and second output ports. The optical combiner has first and second input ports and an output port. The first optical path couples the first output port of the splitter to the first input port of the combiner. The second optical path couples the second output port of the splitter to the second input port of the combiner. Each optical path includes an electro-optical phase shifter, and one of the optical paths includes an electro-optical attenuator. The digital signal generator is configured to apply binary-valued voltage signals to control inputs of the phase shifters and the attenuator.

BACKGROUND

1. Field of the Invention

This invention relates to modulation of optical carriers.

2. Description of the Related Art

Many conventional optical modulators implement binary ON/OFF keyingmodulation schemes. The binary ON/OFF keying modulation schemes encodeone data bit onto an optical carrier per coding interval. While suchmodulation schemes are straightforward to implement, it is oftendesirable to encode more than one data bit onto the optical carrier percoding interval, e.g., to support a higher data rate.

Other conventional optical modulators implement a quadrature phase shiftkeying (QPSK) modulation scheme. The QPSK modulation scheme encodes twodata bits onto an optical carrier per symbol interval thereby producingthe constellation of signal points shown in FIG. 1. In the QPSKconstellation, each signal point has x and y components of the samemagnitude. The various signal points of the QPSK constellation arerelated by reflections about the x-axis and/or the y-axis. In the QPSKmodulation scheme, the optical carrier's in-phase and quadrature-phasecomponents represent the x-coordinates and the y-coordinates of thesignal points.

The conventional QPSK optical modulator includes a Mach-Zehnderinterferometer (MZI). The MZI has two arms whose optical path lengthsdiffer by ¼ of the optical carrier's wavelength up to integer multiplesof the wavelength. Each arm of the MZI includes an electro-opticallycontrolled phase shifter, i.e., an MZI.

The phase shifters generate phase shifts of 0 or 7 on the opticalcarrier in response to the binary voltage values of the digital signalsbeing encoded. In the QPSK optical modulator, one arm of the MZI encodesone data bit onto the sign of the in-phase component of the opticalcarrier, and the other arm of the MZI encodes one data bit onto the signof the quadrature-phase component of the optical carrier.

SUMMARY

Various embodiments provide for quadrature amplitude modulation (QAM) ofoptical carriers. The (QAM) schemes encode more than four signal pointsonto the optical carrier per coding interval.

In one aspect, an apparatus includes an optical splitter, an opticalcombiner, first and second optical paths, and a digital signalgenerator. The optical splitter has an input port and first and secondoutput ports. The optical combiner has first and second input ports andan output port. The first optical path couples the first output port ofthe optical splitter to the first input port of the optical combiner.The second optical path couples the second output port of the opticalsplitter to the second input port of the optical combiner. Each opticalpath includes an electro-optical phase shifter, and one of the opticalpaths includes an electro-optical attenuator. The digital signalgenerator is configured to apply binary-valued voltage signals tocontrol inputs of the phase shifters and the attenuator.

In various embodiments, the electro-optical phase shifter is configuredto function as a binary phase key encoder.

In another aspect, a method includes splitting an input light beam intomutually coherent first and second light beams, modulating the first andsecond light beams, and then, recombining the modulated first and secondlight beams to produce a modulated output light beam. The modulatingencodes, at least, two data bits onto the first light beam per codinginterval and encodes one or more data bits onto the second light beamper the coding interval. The recombining produces a relative phase shiftof π/2 modulo integer multiples of π between the first and second lightbeams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 show a quadrature phase shift keying (QPSK) constellation thatis implemented in prior art optical modulators;

FIG. 2 shows a quadrature amplitude modulation (QAM) constellation thathas sixteen signal points;

FIG. 3 is a block diagram for a system that modulates an optical carrieraccording to a QAM constellation with more than four signal points;

FIG. 4A shows a Mach-Zehnder interferometer for use in a push/pullvoltage-biasing mode;

FIG. 4B shows a phasor diagram that illustrates how push/pull voltagebiasing operates the Mach-Zehnder interferometer of FIG. 4A;

FIG. 5 is a block diagram for a specific embodiment of the opticalmodulator of FIG. 3 in which an optical carrier is modulated accordingto the QAM constellation of FIG. 2;

FIG. 6 is a block diagram for another specific embodiment of the opticalmodulator of FIG. 3 in which an optical carrier is modulated accordingto a QAM constellation with sixty-four signal points; and

FIG. 7 is a flow chart illustrating a method for performing quadratureamplitude modulation on an optical carrier.

Herein, like reference numerals indicate elements with similarfunctions.

Illustrative embodiments are described more fully with reference to theaccompanying figures and detailed description. The inventions may,however, be embodied in various forms and are not limited to embodimentsdescribed herein.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Various embodiments provide for quadrature amplitude modulation (QAM) ofan optical carrier. The modulation schemes encode more than two databits onto the optical carrier per coding interval. The modulationschemes produce optical carriers whose in-phase and quadrature-phasecomponents correspond to the coordinates signal points of QAMconstellations with more than four signal points, i.e., constellationscapable of representing more than 2 bits.

Herein, a coding interval is a time period over which the data on bothin-phase and quadrature-phase components of the optical carrier changes.

FIG. 2 shows a QAM constellation having 16 signal points. This QAMconstellation has signal points at the points (n/3)x+(m/3)y where “n”and “m” are integers selected from {+1, +3, −1, −3}. For this QAMconstellation, the in-phase component of a modulated carrier willcorrespond to the x-coordinate of a signal point and thequadrature-phase component of the modulated carrier will correspond tothe y-coordinate of the signal point. To represent the coordinates ofsignal points of this QAM constellation, the in-phase andquadrature-phase components of the optical carrier should have a phasewith values selected from {0, π} and amplitudes selected from {A, 3A}.

FIG. 3 shows a system 10 that encodes (2N) data bits onto an opticalcarrier during each coding interval where N is an integer equal to orgreater than two. The system 10 includes a digital signal generator 12,a laser 14, and an optical modulator 16. The digital signal generator 12outputs 2N binary-valued voltage signals per coding interval, i.e.,Bit_1, Bit_2, . . . Bit_(N−1), and Bit_2N. For each bit “k”, theassociated voltage signal, Bit_k, takes its voltage values from anappropriate set of two voltages, e.g., {+v, −v}. The laser 14 emits anunmodulated, continuous-wave optical carrier of wavelength, λ. Exemplarylasers 14 include diode lasers. The optical modulator 16 modulates theoptical carrier, which is received from the laser 14 at optical input26, to produce a quadrature amplitude modulated optical carrier atoptical output 28. The optical modulator 16 produces acarrier-modulation that is responsive to the 2N binary-valued voltagesignals output by the signal generator 12 during each coding interval.

Optical modulator 16 has a set of electrical control inputs {CI₁,CI_(2,) . . . CI_(2N-1), CI_(2N)}. Each control input CI₁-CI_(2N)connects to receive one of the binary data signals, which are output bythe digital signal generator 16. Each of the binary data signalfunctions as a binary-valued control voltage for a corresponding one ofthe electro-optical elements in the optical modulator 16, i.e., elements36 ₁-36 _(2N). The electro-optical elements 36 ₁-36 _(2N) of the opticalmodulator 16 are configured so that no analog conversions are needed inorder that the digital data signals produce suitable modulation valueson the optical carrier. Avoiding the need to digital-to-analog convertthe digital data signal voltages, which are output by digital signalgenerator 16, is advantageous at high data rates where digital-to-analogconverters have complex electronic circuits.

The optical modulator 16 includes an electrically controlled,Mach-Zehnder interferometer. The Mach-Zehnder interferometer has a 1×2optical splitter 18, a 2×1 optical combiner 20, and electro-opticallycontrolled optical paths 22, 24. The 1×2 optical splitter 14 separates alight beam at optical input 26 into a first light beam that is directedinto optical path 22 and a mutually coherent second light beam that isdirected into optical path 24. The 2×1 optical combiner 16 recombinesmodulated first and second light beams, which are output by the opticalpaths 22, 24, to produce a modulated output light beam at optical output28. The optical paths 22, 24 include planar or fiber optical waveguidesegments and optical devices 30, 36 ₁-36 _(2N) located between saidsegments. The optical paths 22, 24 modulate light beams therein in amanner that is responsive to binary-valued data voltage signals receivedfrom digital signal generator 12. The optical paths 22, 24 also producea relative time delay between the originally mutually coherent first andsecond light beams so that these light beams have a relative phasedifference of π/2 upon recombination in 2×1 optical combiner 20, e.g.,when no voltages modulate signals on the optical paths 22, 24.Generally, the relative phase difference is π/2 up to an integermultiple of 7. Due to the relative phase difference of π/2, the opticalpath 22 modulates a component of the light beam from laser 14 that isπ/2 out-of-phase with the component modulated by the optical path 24.The relative phase shift π/2 may result from an extra portion of opticalwaveguide 30 and/or a suitable DC voltage applied to one of theelectro-optically controllable waveguide segments of one optical path22, 24.

Each optical path 22, 24 includes an electro-optical phase shifter 36 ₁,36 ₂ and one or more electro-optical attenuators 36 ₃-36 _(2N).

Each electro-optical phase shifter 36 ₁, 36 ₂ includes a control inputCI₁, CI₂ and is responsive to voltages applied to the associated controlinput CI₁, CI₂. In response to the two voltage values of the binary datasignals from signal generator 12, i.e., Bit_1 or Bit_2, theelectro-optical phase shifters 36 ₁, 36 ₂ produce binary phase shift key(BPSK) encoding of data on light at the wavelength of laser 14. That is,the electro-optical phase shifters 36 ₁, 36 ₂ produce phase shifts of 0and π without producing substantial attenuation at the laser'swavelength. For each phase shifter 36 ₁, 36 ₂, the associated binarydata signal takes voltage values appropriate to produce such binaryphase shifting. The construction of the electro-optical phase shifters36 ₁, 36 ₂ causes the specific voltage values of the binary data signalsof signal generator 12 to produce these desirable phase shift values.Nevertheless, the set of voltage values for Bit_1 and Bit_2 may differdue to differences between the two electro-optical phase shifters 36 ₁,36 ₂. Thus, the phase shifts 0 and π are produced withoutdigital-to-analog conversions of digital data signals from the signalgenerator 12. The 0 and π phase shifts are desirable, because theygenerate signal constellations in which signal points are at reflectionsymmetric positions about the x and y axes. The QAM constellation ofFIG. 2 has such reflection symmetries.

Exemplary electro-optical phase shifters 36 ₁, 36 ₂ for optical BPSKencoding include specially configured Mach-Zehnder interferometers asdescribed below with respect to FIGS. 4A-4B. See also, U.S. Pat. No.6,711,308 (Herein, '308 patent.), which is incorporated herein byreference in its entirety.

Each electro-optical attenuator 36 ₃-36 _(2N) includes a control inputCI₃-CI_(2N) and is responsive to voltages applied to the associatedcontrol input CI₃-CI_(2N). In response to the two voltage values ofbinary data signals from signal generator 12, i.e., Bit_3-Bit_2N, eachelectro-optical attenuator 36 ₃-36 _(2N) attenuates light of thewavelength of laser 14 without producing a significant change in phase.For each electro-optical attenuator 36 ₃-36 _(2N), the associated binarydata signal takes voltage values appropriate for desired attenuationvalues. The set of voltage values may be the same for Bit_3-Bit_2N ormay differ for Bit_3-Bit_2N due to differences between the twoelectro-optical attenuators 36 ₃-36 _(2N). Exemplary optical attenuators36 ₃-36 _(2N) include Mach-Zehnder interferometers that are speciallyconfigured to produce substantially pure amplitude attenuations asdescribed below with respect to FIGS. 4A-4B.

Each attenuator 36 _(k) has a transmission coefficient that takes valuesfrom the set {T_(+k), T_(−k)} in response to the respective first andsecond voltage values of the binary data signals transmitted by signalgenerator 12 to the control input CI_(k). Herein, a transmissioncoefficient is a ratio of a received light amplitude to a transmittedlight amplitude at a selected wavelength. The ratios T_(+k)/T_(−k)define the magnitudes of the x and y coordinates of signal points in theQAM constellation generated by system 12.

One embodiment of system 10 modulates the optical carrier according tothe QAM constellation of FIG. 2. In this embodiment, each optical path22, 24 includes a single attenuator 36 ₃, 36 ₄. To produce the QAMconstellation of FIG. 2, the optical attenuators 36 ₃, 36 ₄ areconfigured to have optical transmission coefficients T₊₃, T⁻³, T⁻⁴, andT⁻⁴ whose ratios approximately, e.g., satisfy: T₊₃/T⁻³=T₊₄/T⁻⁴=3. Otherembodiments have ratios satisfying: T⁻³/T₊₃=T₊₄/T⁻⁴=3,T₊₃/T⁻³=T⁻⁴/T₊₄=3, or T⁻³/T₊₃=T⁻⁴/T₊₄=3. In these various embodiments,the equalities for the ratios have errors of less than 25% andpreferably have errors of ±5% or less.

In the embodiment that modulates the optical carrier according to theQAM constellation of FIG. 2, the system 10 includes a digital signalgenerator 12 that outputs 4-bit data signals per coding interval, i.e.,Bit_1, Bit_2, Bit_3, and Bit_4. For each 4-bit data signal, one bit isoutput on an electrical line connecting to one of the control inputs,CI₁-CI₄. Each bit of the 4-bit signal has a voltage value that isselected from a set of the appropriate two voltage values, e.g.,{V_(low), V_(high)}.

FIGS. 4A and 4B illustrate how an exemplary Mach-Zehnder interferometer(MZI) 40 may function as an optical BPSK encoder, e.g., electro-opticalphase shifters 36 ₁, 36 ₂ of FIG. 3, or as an electro-opticalattenuator, e.g., optical attenuators 36 ₃-36 _(2N) of FIG. 3. The MZI40 includes an optical splitter S, an optical combiner C, substantiallyidentical optical paths A₊, A⁻, and substantially identicalelectro-optical control segments E₊, E⁻. Any difference in optical pathlength between optical path A₊ and optical path A⁻ is herein, assumed tobe compensated by a DC bias voltage that is applied to one of theelectro-optical control segments E₊, E⁻. The optical splitter S splitsthe power of an optical input signal equally between two opticaloutputs. The optical combiner C combines optical signals, which arereceived from its two optical inputs, with equal weights to produce anoptical output signal.

To produce an optical BPSK encoder or a pure optical attenuator,electro-optical control segments E₊, E⁻ are operated in push/pull ACbiasing mode. In this mode, electro-optical control segment E₊ receivesa control voltage, +V, and electro-optical control segment E⁻ receives acontrol voltage, −V. That is, control voltages of equal magnitude andopposite sign are applied to the substantially identical electro-opticalcontrol segments E₊ and E⁻. For example, the control voltage of oppositesign, −V, may, e.g., be produced from original control voltage “+V” by avoltage inverter.

FIG. 4B illustrates push/pull mode operation with pairs of phasors P⁺,P⁻. The length and phase of each phasor P⁺, P⁻ of a pair corresponds tothe amplitude and phase of an optical signals produce by one of theoptical paths A₊, A⁻ in push/pull mode operation. Since optical pathsA₊, A⁻ are identical and electro-optical control segments E₊, E⁻ areidentical, each push/pull voltage biasing state produces a pair ofphasors (P⁺, P⁻), wherein |P⁺|=|P⁻| and phase(P⁺)=−phase(P⁻). For thatreason, in the push/pull mode, MZI 40 produces an optical output signal,i.e., P⁺+P⁻, whose phase is either 0 or π.

In push/pull mode, binary control voltage signals can operate MZI 40 asa pure optical attenuator or as an optical BPSK encoder. For operationas a pure optical attenuator, each electro-optical control segment E₊,E⁻ is constructed to respond to the binary voltage values of digitalsignal generator 12 by producing a phase shift of less than π/2 on theoptical signals in the associated optical path A₊, A⁻. In FIG. 4B, thisis illustrated by binary voltage values Vo and ±V₁ and associated pairsof phasors (P⁺ ₀, P⁻ ₀) and (P⁺ ₁, P⁻ ₁). For operation as an opticalBPSK encoder, electro-optical control segment E₊, E⁻ are constructed torespond to the same binary voltages by causing optical signals in theassociated optical paths A₊, A⁻ to undergo phase shifts of 0 and π. InFIG. 4B, this is illustrated by the voltage values Vo and ±Vπ and theassociated phasor pairs (P⁺ ₀, P⁻ ₀) and (P⁺ _(π), P⁻ _(π)).

Other embodiments (not shown) may have single control-electrode opticalmodulators for electro-optical phase shifters 36 ₁, 36 ₂ of FIG. 3,and/or for electro-optical attenuators 36 ₃-36 _(2N) of FIG. 3 ratherthan the above-described two control-electrode MZIs 40, which areoperated in push/pull mode. For example, a single control-electrodeoptical modulator that can produce opposite phase shifts in twoneighboring optical waveguides is described in U.S. Pat. No. 5,050,948,which is incorporated herein by reference in its entirety.

FIG. 5 shows an embodiment 16A of the optical modulator 16 of FIG. 3that encodes according to the QAM constellation of FIG. 2. In theoptical modulator 16A, each electro-optical phase shifter and attenuator36 ₁-36 ₄ includes a Mach-Zehnder interferometer with a 1×2 opticalsplitter S, a 2×1 optical combiner C, and two substantially identicaloptical paths, i.e., A₊′ and A⁻′ or A₊″ and A⁻″. The substantiallyidentical optical paths also include substantially identical controlsegments, i.e., E⁻⁺′ and E⁻′ or E⁻⁺″ and E⁻″, in which refractiveindexes respond to voltages applied to associated control inputsCI₁-CI₄.

In embodiment 16A, signal generator 12 connects to control segments E₊′and E⁻′ an electro-optical phase shifter 36 ₁, 36 ₂ and to controlsegments control segments E₊″ and E⁻″ of an electro-optical attenuator36 ₃, 36 ₄ in a manner that produces push/pull mode operation. In thepush/pull mode, each pair of control segments E₊′ and E⁻′ receiveopposite sign AC control voltages, and each pair of control segments E₊″and E⁻″ receive opposite sign AC control voltages. The relative signinversions may result from connecting two binary outputs of the signalgenerator 12 directly to the control segment E₊″ and E₊′ whileconnecting the same binary outputs to a voltage inverter whose outputconnects to the corresponding paired control segment E⁻″ and E⁻′. Sincethe paired optical paths A₊′ and A⁻′ are substantially identical, suchpush/pull application of control voltages ensures that phase shifters 36₁, 36 ₂ cause phase shifts of 0 and/or π. The electro-optical phaseshifters 36 ₁, 36 ₂ are specifically constructed to cause phase shiftsof 0 and π to light of the wavelength of laser 14 in response toreceiving the binary voltage values produced by digital signal generator12. Similarly, since the paired optical paths A₊″, A⁻″ are substantiallyidentical, such push/pull application of control voltages ensures thatthe electro-optical attenuators 36 ₃, 36 ₄ cause phase shifts of 0and/or π. The electro-optical attenuators 36 ₃, 36 ₄ are specificallyconstructed to cause phase shifts of 0 on light of the wavelengthproduced by the laser 14 in response to receiving the binary voltagevalues produced by the digital signal generator 12. Thus, the MZIs ofthe phase shifters 36 ₁, 36 ₂ are constructed to function as opticalBPSK encoders for the binary voltage values output by the digital signalgenerator 12, and the MZIs of the electro-optical attenuators 36 ₃, 36 ₄are constructed differently so that they function as pure opticalattenuators in response to the same binary voltage values.

In other embodiments (not shown), optical modulator 16A is modified byremoving optical attenuator 36 ₄ and by connecting the optical output ofelectro-optical phase shifter 36 ₂ directly to one of the optical inputsof optical splitter 20. Such embodiments of optical modulator 16 of FIG.3 produce an embodiment of system 10 in which 3-bits of digital data areencoded onto the optical carrier per coding interval, i.e., Bit_1,Bit_2, and Bit_3.

FIG. 6 shows another specific embodiment 16B of optical modulator 16 ofFIG. 3. The optical modulator 16B is used by an embodiment of system 10that modulates the optical carrier according to a QAM constellationhaving sixty-four signal points, i.e., N=3. In the optical modulator16B, each optical path 22, 24 includes a pair of optical attenuators,i.e., (36 ₃, 36 ₅) or (36 ₄, 36 ₆). Each electro-optical attenuator 36₃-36 ₆ is controlled by a corresponding single bit of digital datasignal output by signal generator 12. Again, each electro-opticalattenuator 36 ₃-36 ₆ includes a MZI with a pair of substantiallyidentical optical paths, i.e., (A₊″, A⁻″). In these MZIs, the pairedoptical paths (A₊″, A⁻″) have substantially identical control segmentsE⁻⁺″, E⁻″. Again, the digital signal generator 12 operates each of theseMZIs in a push/pull mode. Thus, in response to the voltage signals fromdigital signal generator 12, the electro-optical attenuators 36 ₃-36 ₆attenuate optical carriers at the wavelength of laser 14 withoutproducing significant phase shifts thereon.

FIG. 7 illustrates a method 50 for modulating an optical carrier tocarry more than two bits of data per coding interval, e.g., usingoptical modulator 16, 16A, 16B of FIG. 3, 5, or 6. The method 50produces, e.g., a quadrature modulated optical carrier whose signalpoints belong to a QAM constellation.

The method 50 includes splitting a coherent light beam into mutuallycoherent first and second light beams (step 52). After the splitting,light of the first and second light beams propagates along respectivefirst and second optical paths.

The method 50 includes modulating the first light beam to carry one setof data bits during a coding interval and modulating the second lightbeam to carry another set of data bits during the same coding interval(step 54). For each of the first and second light beams, the modulatingstep includes a step of electro-optically modulating the phase of thelight beam to carry one data bit of the associated set. The step ofelectro-optically modulating the phase produces a phase shift of 0 or πin a manner responsive to the value of the associated data bit, i.e.,optical BPSK encoding.

For one or both of the first and second light beams, the modulating stepincludes a step of electro-optically modulating the amplitude of thelight beam to carry the remaining one or more data bits of theassociated set. The step of electro-optically modulating the amplitudeproduces an amplitude belonging to a set of values. In an embodiment ofmethod 50 that produces the QAM constellation of FIG. 2, the twoamplitude values have a ratio of 3.

The method 50 also includes then, recombining the modulating first andsecond light beams, e.g., coherently recombining, with a relative phaseshift of π/2 to produce an output modulated light beam (step 56). Therelative phase shift ensures that the quadrature component of the outputlight beam carries ½ of the data bits of a coding interval and that thein-phase component of the output light beam carries the remaining ½ ofthe data bits of the coding interval.

Other embodiments of the invention will be apparent to those of skill inthe art in light of the specification, drawings, and claims of thisapplication.

1. An apparatus, comprising: an optical splitter having an input portand first and second output ports; an optical combiner having first andsecond input ports and an output port; a first optical path coupling thefirst output port of the optical splitter to the first input port of theoptical combiner; a second optical path coupling the second output portof the optical splitter to the second input port of the opticalcombiner; and a digital signal generator; wherein each optical pathincludes an electro-optical phase shifter and one of the optical pathsincludes an electro-optical attenuator; and wherein the digital signalgenerator is configured to apply binary-valued voltage signals tocontrol inputs of the phase shifters and the attenuator.
 2. Theapparatus of claim 1, wherein the electro-optical phase shifter isconfigured to function as a binary phase key encoder.
 3. The apparatusof claim 2, wherein the other of the optical paths includes anelectro-optical attenuator and the digital signal generator isconfigured to apply binary-valued voltage signals to a control input ofthe attenuator of the other of the optical paths.
 4. The apparatus ofclaim 3, wherein the signal generator is configured to apply firstbinary-valued voltage signals to the control input of the encoder ofeach path and to apply second binary-valued voltage signals to thecontrol input of the attenuator of each path.
 5. The apparatus of claim3, wherein the apparatus is configured to modulate an optical carrierreceived at the input of the optical splitter according to a QAMconstellation in response to the signals applied by the signalgenerator.
 6. The apparatus of claim 5, wherein the QAM constellationhas 16 signal points.
 7. The apparatus of claim 3, further comprising alaser positioned to transmit laser light to the input port of theoptical splitter; and wherein in response to said digital voltagesignals, each electro-optical phase shifters is configured to phaseshift received light from said laser light between two values thatdiffer by about R.
 8. The apparatus of claim 3, wherein the first andsecond optical paths are configured such that light beams sent by theoptical splitter to the first and second optical paths receives arelative phase shift of about π/2 up to integer multiples of π betweenbeing produced in the optical splitter and being recombined in theoptical combiner.
 9. A method for modulating an optical carrier,comprising: splitting an input light beam into mutually coherent firstand second light beams; modulating the first light beam to encode, atleast, two data bits thereon per coding interval; modulating the secondlight beam to encode one or more data bits thereon per the codinginterval; and then, recombining the modulated first and second lightbeams to produce a modulated output light beam, the recombiningproducing a relative phase shift of π/2 modulo integer multiples of πbetween the first and second light beams.
 10. The method of claim 9,wherein the modulating is such that a value of a first of the data bitsdetermines a modulated phase of the one of the first and second lightbeams during the coding interval and a value of a second of the databits determines a modulated amplitude of the one of the first and secondlight beams during the coding interval.
 11. The method of claim 10,wherein the modulating is such that a value of a third of the data bitsdetermines a modulated phase of the other of the first and second lightbeams during the coding interval and a value of a fourth of the databits determines a modulated amplitude of the other of the first andsecond light beams during the coding interval.
 12. The method of 10,wherein the two values of the first of the data bits modulate the phaseof the one of the first and second light beams by phases that differ byabout π.
 13. The method of claim 9, wherein the output light beam is aquadrature amplitude modulated version of the light beam.
 14. The methodof claim 13, wherein the modulated output light beam is quadratureamplitude modulated according to a QAM constellation with 16 signalpoints.